Continuous bluing and annealing process



Mrch 7, 1967 I r. G. KATSAHNIAS 3,307,981

CONTINUOUS BLUING AND ANNEALING PROCESS Filed Nov. 14,1963 2 Sheet-Sheet 1 \O I m 4) United States Patent 3,307,981 CONTINUUUS BLUING AND ANNEALING PROCESS Thomas G. Katsahnias, South Holland, 111., assignor to Inland Steel Company, Chicago, lllL, a corporation of Delaware Filed Nov. 14, 1963, Ser. No. 323,802

6 (Claims. (Cl. 148-6.35)

The present invention relates generally to a method of continuously bluing a ferrous metal strip, and more particularly to an improved method of continuously bluing a ferrous metal strip on continuous annealing apparatus.

Various methods of continuously bluing a ferrous metal strip have been devised in which a gas-containing unc-ombined oxygen or steam have been introduced into a particular zone within a continuous annealing apparatus in order to oxidize or blue the surface of a ferrous metal strip. In general, the oxide films produced have been relatively thin, and it has been necessary to make extensive modifications in the equipment in order to introduce the oxygen-containing gas at a particular zone within the annealing furnace. Also, it has generally been necessary to reduce the rate of travel of the ferrous metal strip below the normal line speed in the annealing apparatus.

It is therefore an object of the present invention to provide an improved method of continuously bluing a ferrous metal strip which is adapted to provide a desirable oxide film in a more economical manner.

. It is another object of the present invention to provide unimproved method of continuously bluing a ferrous metal strip while passing the strip through continuously annealing apparatus at the high line speeds at which metal strips are normally processed on continuous annealing apparatus.

It is a further object of the present invention to provide an improved method of continuously bluing a ferrous metal strip with conventional continuous annealing apparatus without making extensive alterations therein.

Other objects of the present invention will be apparent from the detailed description and claims which follow when read in conjunction with the accompanying drawing, wherein:

FIG. 1 is a schematic vertical sectional view of a continuous annealing furnace in which the process of the present invention can be practiced;

FIG. 2 is a fragmentary elevational view taken along the line 2-2 of FIG. 1; and

FIG. 3 is a fragmentary schematic vertical sectional view of a modified continuous annealing furnace in which a modified form of the invention can be practiced.

Heretofore it has been considered necessary from an operational and safety standpoint when using continuous annealing apparatus for bluing an endless strip of ferrous metal to operate the continuous annealing furnace at a pressure greater than atmospheric pressure so that ambient air is not drawn into the interior of the furnace. Thus, whenever air or another oxygen-containing gas has been introduced into a continuous annealing furnace for bluing the metal strip during the annealing treatment, the oxidizing gas required has been introduced directly into the furnace heating zone through auxiliary pipes or headers communicating with the desired section of the annealing furnace while maintaining the interior of the furnace at a super atmospheric pressure by supplying simultaneously thereto large volumes of non-oxidizing furnace gas. It has now been discovered, however, that it is possible to safely and efficiently operate a continuous annealing furance for simultaneously annealing and bluing a ferrous metal strip by maintaining the pressure within ICC the annealing furnace below the ambient atmospheric pressure so that a relatively large volume of low veloc1ty atmospheric air is drawn into the annealing furnace through the entry seal rolls; whereby the air first impinges on the strip at a point where the strip is at a low temperature (i.e. room temperature or slightly above) and is available to effect the desired degree of surface oxidation or bluing of the metal strip in the heating zone adjacent to the entry seal rolls. By regulating the degree of negative or sub-atmospheric pressure within the continuous annealing furnace, and particularly within the heating zone which has one end thereof closed by the entry seal rolls, it is possible to control the amount of surface oxidation of the ferrous metal strip and provide a highly satisfactory oxide coating within a wide range of oxide coating weights.

A further important element of the present invention and one which contributes significantly to the capability of the present invention producing substantial heavier oxide coating Weights in a short period of time is the control exercised over the moisture content of the furnace gas atmosphere. Thus, it has been found that by controlling the dew point of the furnace atmosphere so that the dew point is above 0 F. and below that at which condensation forms within the furnace chamber, and preferably between about +20 F. and +50 F., unusually good oxide coatings are produced rapidly throughout the entire range of coating weights between about 1 gram and 4 grams per square foot and without causing damage to the continuous annealing furnace. Without the foregoing control of the dew point only relative low oxide coating weights can be produced when operating at the high line speeds employed in the present process which can, if desired, be in excess of 1000 ft. per minute.

The reduction of the pressure of the furnace atmosphere at the inlet end of and within the heating zone of the furnace below atmospheric pressure can be effected by either of two procedures which has been found highly of the infiowing non-oxidizing furnace gas below that which is normally introduced into the furnace (when maintaining a positive pressure therein) without restricting the normal outflow means for the heated gases from the furnace which normally comprise the outlet seal rolls of the furnace. For example, a continuous annealing furnace having a volume of about 27,000 cu. ft. is normally supplied with between about 30,000 to 35,000 cu. ft. per hour of a non-oxidizing gas which will maintain the pressure within the annealing furnace greater than atmospheric pressure. When, in accordance with the practice of the present invention, a volume of non-oxidizing furnace gas, such as a mixture of 310% H and 9790% N is supplied to the annealing furnace in an amount between about 10,000 cu. ft. and about 15,000 cu. ft. per hour by means of the usual entry ports, the pressure within the annealing furnace and the heating zone thereof falls below atmospheric pressure. The foregoing assumes, of course, that there are no appreciable leaks in the radiant heating tubes, for example, through which gas would be withdrawn from the furnace. When the pressure within the furnace in the heating zone is below atmospheric pressure, a substantial volume of low velocity air-containing free oxygen is drawn into a relatively low temperature zone of the annealing furnace through the inlet or entry seal rolls proximate to the heating zone of the annealing furnace but at a point relative to the metal strip where the temperature of the strip is about room temperature or slightly above and well below its surface oxidizing temperature. The air containing oxygen travels a with the strip into the interior of the heating zone where oxidation or bluing of the strip takes place.

An alternate or auxiliary method of reducing the pressure within the interior of the continuous annealing furnace is to employ one or more exhaust'gas outlets with an exhaust blower or a plurality of exhaust blowers communicating with the interior of the annealing furnace and preferably with the hottest section of the heating zone. When the rate of withdrawal of the atmosphere within the furnace is increased by the exhaust blowers while the supply of furnace gas remains constant or is reduced, it will be evident that a volume of oxygencontaining air Will be drawn into the annealing furnace through the entry seal rolls and into the interior of the heating zone of the furnace where oxidation or bluing of the strip takes place.

In each of the foregoing methods, the pressure within the continuous annealing furnace and the degree of oxidation of the strip is controlled by either (1) increasing or decreasing the amount of non-oxidizing gas supplied to the annealing furnace, (2) by increasing or decreasing the exhaust blower output or (3) by a combination of l) and (2). For example, if it is desired to produce a heavier oxide coating on the ferrous metal strip, the amount with the oxygen supplied by the atmospheric air or moisture in the gas entering the furnace.

The moisture content of the continuous annealing furnace atmosphere which is required in the present process is established and controlled by first adjusting the furnace atmosphere before the bluing run commences to a hydrogen content of preferably between about 3% to 6% with the balance being substantially nitrogen. Thereafter the pressure within the furnace is reduced in any of the above described methods or combination thereof to permit air to enter through the entry seal rolls. After the atmosphere within the furnace has come to equilibrium, generally in about 5 minutes, the moisture content thereof is adjusted or maintained at the desired level by supplying either high dew point atmospheric air or by regulating the dew point of the incoming furnace gas. Generally, it is considered preferable to control the dew point of the furnace gas, since the dew point of the furnace gas can be more conveniently raised or lowered to compensate for the low or high moisture content of the atmospheric air.

The following Table I sets forth the operating conditions and data illustrating the process of the present in- Vention:

T ABLE I Run A B 0 Strip Size (in) 24. 37 x 0143 24. x .0136 24. 24 x .0136 Line Speed (it nin.) 450 600 475 Volume of HNX Gas Input (cu. ft./hr.) 12, 000 11,800 11, 200 Furnace Atmosphere at Equilibrium:

l. 0 l. 2 1. 1 1. 5 l. 3 l. 0 07. 5 97. 5 07. 9 Dew Point of Incomi C +25 +20 +18 Percent H O by Volume in H X .450 .375 .330 Dew Point of Incoming Air +40 +20 Percent E 0 by Volume of Incoming Air .820 .550 .375 Furnace Pressure (inches H O) 0. 02 0. 05 0. 01 Oxide Coating Weights (gi'au1s 'ft. 3. 0 3.9 1. 2

of hydrogen-nitrogen gas being supplied to the annealing furnace is decreased and/ or the exhaust blower output is increased to thereby lower the pressure within the furnace and cause a greater volume of oxygen-containing air to enter through the inlet or entry seal rolls. If a lighter oxide coating is desired the pressure within the furnace is allowed to increase until the furnace pressure is only slightly below atmosphere by increasing the inflow of nonoxidizing furnace gas to the furnace and/ or by decreasing the exhaust blower output.

It should also be understood that the pressure within the continuous annealing furnace is dependent to some degree on the surface area of the steel strip which is exposed to the furnace atmosphere in any given period of time. Thus, the pressure Within the furnace for a given input of hydrogen-nitrogen gas varies with the line speed of the ferrous metal strip. For example, the faster the line speed of the strip, the more surface must be coated (i.e., oxidized) per unit of time, and the more oxygen is required to oxidize the strip. Consequently, a greater volume of air is required to supply the needed oxygen. Since the volume of air entering the annealing furnace is controlled by the pressure within the interior of the furnace, the pressure within the furnace should be lowered when the line speed is increased in order to maintain a given oxide coating weight.

While the preferred furnace gas supplied to the annealing or normalizing furnace is a reducing gas containing between about 310% hydrogen and 97-90% nitrogen (i.e., HNX gas), it is possible to use other non-oxidizing gases. Since oxygen is admixed with the furnace gas in the process of the present invention to effect the desired bluing, however, consideration must be given to the combutible content of the furnace gas before it is used in this process, as those Skilled in the art appreciate that some critical combustible Ontents may react violently The data set forth in Table I was obtained using commercial tower type continuous annealing apparatus which is illustrated schematically in FIG. 1 of the drawing, wherein a ferrous metal strip 10 having the indicated thickness and width was passed into the interior of the tower type continuous annealing furnace 11 through a pair of oppositely disposed conventional entry seal rolls 12 which have a length of about 56 inches and are spaced about of an inch (see FIG. 3). The interior of the furnace 11 is constructed of refractory material supported by a structural metal frame 13. A series of parallel internal walls 14 of refractory material define a number of parallel vertical pass chambers within the interior of the furnace 11 through which strip 10 is progressively moved. Thus, the strip it) enters the furnace 11 and passes into a heating zone comprising a first pass chamber 15, around a furnace roll 16, through a second pass chamber 17, around a furnace roll 18, through a third pass chamber 19, around a furnace roll 20, through a fourth pass chamber 21, around a furnace roll 22, through a pass chamber 23, around a furnace roll 24, through a pass chamber 25, and around furnace rolls 26, 27 and 28, and enters a pass chamber 29 in an adjacent holding or soaking zone' of the annealing furnace. The strip passes through the various chambers 29, 30, 31, 32, 33 and 34 which comprise the holding zone, after which the strip passes into an adjacent cooling section of the annealing furnace having a plurality (i.e., 20) of the pass chambers 3554 during which the strip 10 is cooled stepwise to a temperature of about 300 F. Thereafter the strip leaves the annealing furnace through the outlet seal rolls through a water quench tank 65, after which it is formed into a suitable coil in a conventional manner.

The annealing furnace shown in FIG. 1 is substantially sealed against the entry of atmospheric air between the entry seal rolls 12 and the exit seal rolls 60'. In general,

a non-oxidizing atmosphere is supplied to the interior of the annealing furnace and is maintained non-reducing with respect to the oxide coating formed on the surface of the ferrous metal strip. The non-oxidizing atmosphere is supplied to the annealing furnace through suitable conventional inlet pipes 5% which are in communication with a suitable furnace gas generator machine of any desired type.

The furnace pass chambers 1525 and 29%! are provided with conventional radiant heat tubes 37 and 38, respectively, although other suitable types of heating elements or means can be provided, if desired. In furnace pass chambers 15 through 25 which comprise the furnace heating zone, the heating tubes are adapted to rapidly heat the strip to a suitable annealing temperature to effect the desired annealing treatment while the strip 10 passes therethrough. The heating elements in pass chambers 29-34 which comprise the holding zone are adapted simply to maintain or hold the temperature of the strip 10 at a predetermined value without further raising the temperature of the strip 10. In actual operation, the ferrous metal strip 10 entering the furnace 11 through the entry seal rolls 12 generally has a temperature of about 70 F. or slightly higher, and thereafter the strip 10' is rapidly raised to a temperature between about 1000 F. and about 1450 F. before the strip leaves pass chamber 25. While the strip 10 remains in pass chambers 2934, the holding temperature is relatively constant and remains at about 1400 F. After leaving chamber 34, the strip 10 enters a series of pass chambers having cooling means associated therewith so that the strip is gradually cooled to a temperature of about 300 F. before leaving the annealing furnace through the exit seal rolls 60.

In the embodiments of the invention described in Table I, oxidation or bluing preferably takes place in pass chambers 19 and 21 While the temperature of the strip is at about 1200 F. It will be understood, of course, that with the heating elements adjusted differently, it is possible to have the oxidation or bluing take place at an earlier or later pass chamber of the heating zone, if desired. In the present invention, however, all of the oxidation occurs in the heating zone while the temperature of the strip is substantially above 1000 F. and preferably at about 1200 F.

In FIG. 2 of the drawing showing a slightly modified form of the apparatus of FIG. 1, the apparatus is the same as shown in FIG. 1 except that the pass chambers 15, 17, 19, 21, 23 and 25 which form the heating zone of the furnace are provided with one or more exhaust outlets 61 having an exhaust blower 62 associated therewith for the removal of furnace atmosphere from the heating zone to facilitate controlling the pressure within the annealing furnace.

In the preceding specification and claims which follow the term bluing is used to designate surface oxidation of the ferrous metal strip to provide a thin oxide coating having either a blue or a black appearing color, as desired.

Others may practice the invention in any of the numerous ways which are suggested to one skilled in the art by this disclosure, and all such practice of invention are considered to be a part hereof which fall within the scope of the appended claims.

I claim:

1. A process of continuously oxidizing the surface of a ferrous metal strip to effect bluing thereof which comprises; passing an endless ferrous metal strip into a continuous annealing furnace chamber through a strip entry passage, guiding said strip within said furnace chamber through a heating zone and a cooling zone, said strip being raised to an annealing temperature within said heating zone, introducing into said furnace chamber a nonoxidizing furnace gas in an amount insufiicient to equal the amount of gas leaving said furnace chamber whereby the pressure Within said furnace heating zone is below atmospheric pressure, drawing oxygen-containing air into said furnace chamber through said strip entry passage to provide a furnace atmosphere containing free oxygen in said heating zone thereof, maintaining said furnace atmosphere at a dew point above 0 F. and below that at which condensation forms within said furnace chamber, and heating said strip while exposed to said furnace atmosphere in said heating zone to a temperature between about 1000 F. and 1450 F.; whereby the surface of the said strip is oxidized to provide thereon a firmly adherent oxide coating effectively bluing the said strip.

2. A process as in claim 1, wherein said furnace atmosphere has a dew point between about ;+20 F. and +50 F. and the temperature of said strip in said heating zone is about 1200 F.

3. A process as in claim 1, wherein said non-oxidizing furnace gas is comprised essentially of a hydrogen-nitrogen mixture containing between about 3% and 10% hydrogen.

4-. A process of continuously oxidizing the surface of a ferrous metal strip to effect bluing which comprises; passing an endless ferrous metal strip into a continuous annealing furnace chamber through a strip entry passage, guiding said strip within said furnace chamber through a heating zone and a cooling zone, withdrawing gases from said heating zone by mechanical means, introducing into said furnace chamber a non-oxidizing gas in an amout insufficient to equal the volume of gas leaving said furnace chamber whereby the pressure within said heating zone is below atmospheric pressure, drawing oxygen-containing air into said chamber through said entry passage to provide a furnace atmosphere containing free oxygen in said heating zone thereof, maintaining said furnace atmosphere at a dew point about 0 F. and below that at which condensation forms within said fur nace atmosphere and heating said strip while exposed to said furnace atmosphere in said heating zone to a tem perature between about 1000 F. and 1450 F.; whereby the surface of said strip is oxidized to provide thereon a firmly adherent oxide coating effectively bluing the said strip.

5. A process as in claim 4, wherein said dew point of said furnace atmosphere is between about +20 F. and +50 F. and the temperature to which said strip is heated in said oxygen-containing furnace atmosphere is about 1200 F.

6. A process as in claim 4, wherein said non-oxidizing furnace gas is comprised essentially of a hydrogen-nitrogen gas mixture, containing between about 3% and 10% hydrogen.

References Cited by the Examiner UNITED STATES PATENTS 2,768,916 10/1956 Seabold et al 148-6.3S

ALFRED L. LEAVITT, Primary Examiner. R. S. KENDALL, Assistant Examiner, 

1. A PROCESS OF CONTINUOUSLY OXIDIZING THE SURFACE OF A FERROUS METAL STRIP TO EFFECT BLUING THEREOF WHICH COMPRIES; PASSING AN ENDLESS FERROUS METAL STRIP INTO A CONTINUOUS ANNELAING FURNACE CHAMBER THROUGH A STRIP ENTRY PASSAGE, GUIDING SAID STRIP WITHIN SAID FURNACE CHAMBER THROUGH A HEATING ZONE AND A COOLING ZONE, SAID STRIP BEING RAISED TO AN ANNEALING TEMPERATURE WITHIN SAID HEATING ZONE, INTRODUCING INTO SAID FURNACE CHAMBER A NONOXIDIZING FURNACE GAS IN AN AMOUNT INSUFFICIENT TO EQUAL THE AMOUNT OF GAS LEAVING SAID FURNACE CHAMBER WHEREBY THE PRESSURE WITHIN SAID FURNACE HEATING ZONE IS BELOW ATMOSPHERIC PRESSURE, DRAWING OXYGEN-CONTAINING AIR INTO SAID FURNACE CHAMBER THROUGH SAID STRIP ENTRY PASSAGE TO PROVIDE A FURNACE ATMOSPHERE CONTAINING FREE OXYGEN IN THE HEATING ZONE THEREOF, MAINTAINING SAID FURNACE ATMOSPHERE AT A DEW POINT ABOVE 0*F. AND BELOW THAT AT WHICH CONDENSATION FORMS WITHIN SAID FURNACE CHAMBER, AND HEATING SAID STRIP WHILE EXPOSED TO SAID FURNACE ATMOSPHERE IN SAID HEATING ZONE TO A TEMPERATURE BETWEEN ABOUT 1000*F. AND 1450*F.; WHEREBY THE SURFACE OF THE SAID STRIP IS OXIDIZED TO PROVIDE THEREON A FIRMLY ADHERENT OXIDE COATING EFFECTIVELY BLUING THE SAID STRIP. 